In order to achieve better channel utilization and increase overall performance, multiple transmission and multiple reception antennas (also commonly referred to as multiple input, multiple output (MIMO)) at both enhanced Node B (eNB) (or base station (BS), Node B (NB), and so forth) and User Equipment (UE) (or mobile station (MS), terminal (TE), and so on) are considered.
An extension to MIMO makes use of multiple NBs to transmit to a single UE or a group of UEs, with the transmissions made by the NBs being coordinated so that the UE or the group of UEs may be able to either combine the transmissions made by the NBs or avoid interference to improve overall performance. The extension is commonly referred to as coordinated multiple point transmission (CoMP). Other coordination schemes involving multiple NBs, such as various time and/or frequency domain reuse and/or power control schemes, may be known as fractional/soft reuse, or interference coordination schemes, or muting, is often considered to be a category of CoMP operation.
Generally, to have better spectral efficiency, a transmitter in a MIMO system needs to know information regarding channels between transmitter and receiver antennas. One commonly used technique is to have a receiver feedback channel information to the transmitter. For example, the receiver may measure the channel between it and the transmitter and feed information regarding the channel back to the transmitter. Channel information may include information such as precoding matrix indicator (PMI) which may be an indication of the precoding to be used, channel state information (CSI) which may be information pertaining to the measured long term or short term channel, channel quality indicator (CQI) which may be an indication of the measured quality of the channel, rank indicator (RI) which may be an indication of a number of spatial layers that can be supported by the channel, and so forth.
When a communications system is using CoMP to help improve overall efficiency, a receiver may need to transmit channel information to multiple transmitters, which the channel information for each transmitter comprising channel information for each of the transmitters transmit antennas and the receivers receive antennas. Therefore, as the number of NBs used in CoMP increase, the amount of feedback overhead can also dramatically increase.
Typically, the feeding back of channel information may be categorized into one of two groups: explicit feedback and implicit feedback. In explicit feedback the receiver provides channel information to the transmitter in an un-encoded (raw) form or with little processing, while in implicit feedback the receiver, given a set of assumptions made by the transmitter and/or the receiver, provides a processed or encoded form of the channel information or a subset of the channel information to the transmitter to help reduce the amount of feedback overhead.
Explicit feedback may result in a significant amount of feedback information to transmit, potentially making explicit feedback a poor candidate when feedback overhead is considered. However, the form of MIMO/CoMP being used may have an impact on the type and amount of feedback information that the receiver needs to provide. Furthermore, there may be an extremely large number of MIMO and CoMP combinations. Therefore, it may be impractical for a communications system to support all combinations of MIMO and CoMP.
Therefore, there is a need for a feedback information framework that facilitates large variations of potential MIMO/CoMP techniques while keeping feedback overhead and complexity low.